In recent years, there has been an increased focus on reducing emissions of greenhouse gases generated by burning fossil fuels. One solution for reducing greenhouse gas emissions is developing renewable sources of energy. Particularly, energy derived from the wind has proven to be an environmentally safe and reliable source of energy, which can reduce dependence on fossil fuels.
Energy in wind can be captured by a wind turbine, which is a rotating machine that converts the kinetic energy of the wind into mechanical energy, and the mechanical energy subsequently into electrical power. Common horizontal-axis wind turbines include a tower, a nacelle located at the apex of the tower, and a rotor that is supported in the nacelle by means of a shaft. The rotor is provided with one or more blades, which can be set into rotation about the shaft by the force exerted by the wind.
In order to increase the amount of wind power intercepted by a wind turbine, the trend in modern wind turbine design is to increase the length of the blades. For example, blades used in modern installations may be up to 80 m long and have a diameter of 5 m and more at the root side.
Given these considerable dimensions, transportation of turbine blades from the production site to the installation site may in some cases be problematic. For long blades, transportation is usually carried out by using vehicles such as that shown in FIG. 1. Vehicle 1 comprises a front tractive unit 3 and a rear non-tractive unit 5. The blade 2 is suspended between the front transportation unit 3 and the rear transportation unit 5 and secured thereto. When the blade 2 is not present in vehicle 1, the rear non-tractive unit 5 is not directly connected to the front tractive unit 3. Rather, the tractive force is transferred from the front tractive unit 3 to the rear non-tractive 5 unit through the blade 2.
Usually, a clamping system is used in order to fasten the tip of the blade 2 to non-tractive unit 5 of vehicle 1. In this case, it is crucial that the clamping force is known and can be adjusted to a desired value. In particular, the clamping force must be high enough for blade 2 to be able to pull rear non-tractive unit 5 without sliding out of the clamp when tractive unit 3 is in motion. On the other hand, the magnitude of the clamping force cannot be increased at will, since too high a clamping force would cause damage or break of the blade surface.
European patent number 2 105 349 B1 describes an example of a transportation unit which can be used as a rear non-tractive unit in the vehicle shown in FIG. 1. With reference to FIG. 2, non-tractive unit 5 comprises a carrier arrangement 26 secured on a rotatable part 25 on a platform 28. The rear end of a wind turbine blade 2 may be fixed to a plurality of fixing frames 27. The blade 2 is secured to a fixing frame 27 by fixing a top horizontal rod 29 to a pair of vertical rods 30 laterally delimiting fixing frame 27. However, in the non-tractive unit described by EP 2 105 349 B1, the clamping force exerted on the blade can neither be precisely known, nor be adjusted to a predetermined value.
Although extremely important, adjusting the clamping force to a predetermined value is particularly challenging. Due to manufacturing tolerances on both the clamps and the blades, the position of the clamp jaws must be adjusted from blade to blade, in order to ensure that the clamping force meets the required value.
Furthermore, a reliable device for enabling clamping force adjustment can neither rely on, nor involve any frictional forces. For example, one might consider using a screw or a bolt which could be tightened with respect to a cooperating nut by applying a predetermined moment. However, the clamping force would in that case depend on the friction between the thread on the bolt and the cooperating thread on the nut. This friction can dramatically vary depending on many factors. For example, if the screw or bolt is new and duly greased, the friction is in general different from that of the same system after a certain amount of time. Moreover, given a fixed time interval, for example one year, the friction will in general vary in different manners depending on external factors such as the environmental conditions where the clamp is. Thus, if the clamp has been mainly stored in an indoor facility, the variation of friction between the nut and the bolt is different from the case in which the nut has been mainly kept outdoors during the time interval considered. In all cases, this unpredictable change of friction between the bolt and the nut will be directly transferred to the clamping force, which therefore cannot be set to a known, desired magnitude.
For analogous reasons, it is not recommended to use one or a stack of Belleville spring washers in the clamp in order to directly or indirectly apply an elastic force to the blade. Belleville spring washers are known to display a hysteresis cycle in their load vs. deflection characteristic curve mainly caused by the friction between the spring washer and the loading surfaces. This hysteresis effect is enhanced when more Belleville spring washers are stacked in parallel. Thus, given a load applied to a single Belleville spring washer or a system thereof, the deflection cannot in general be determined in advance without knowing the previous history of the system. Symmetrically, given a predetermined value of deflection of each Belleville spring washer in a stack, the elastic force cannot be univocally determined.
Furthermore, the friction between adjacent spring washers or of the spring washer with the loading surfaces may vary over time. This is especially the case if spring washers are used in clamps which are mainly stored outdoors.
Thus, if Belleville spring washers are used in clamping systems for wind turbine blades, they must be frequently replaced so that the required clamping force can be achieved for all blades. This solution is clearly unsatisfactory due to the high costs required and limited possibilities of controlling the clamping force, especially over time.
In view of the problems and drawbacks noted above, a need exists for an improved clamping system adapted to be used when storing and transporting a wind turbine blade, particularly a blade of huge dimensions. More specifically, a need exists in the state of the art for a clamping system for a wind turbine blade, wherein the magnitude of the clamping force can be reliably and precisely known. Furthermore, one more need in the field of wind turbine blade manufacturing is that for a clamping system for a wind turbine blade, wherein the magnitude of the clamping force may be adjusted at will.